WASHINGTON An asteroid big enough to wipe out cities whizzedpast the Earth last month before astronomers even knew it was there,showing how hard it is to forecast a natural disaster from space, aNASA scientist said Thursday.
The asteroid, a collection of rock and dust a half-mile or morein diameter, flashed by at a speed of 46,000 m.p.h. and was not evendetected until days after it had become only a receding point oflight, the National Aeronautics and Space Administration announced.
Astronomers reported that the object missed Earth by only ahalf-million miles - about twice the distance between the Earth andmoon. In cosmic terms, this is considered "a close call."
Had it hit the Earth, said Bevan French, a scientist in NASA'ssolar system exploration division, the asteroid would have created acrater the size of the District of Columbia and spread destructionfor 100 miles in all directions. Other effects would be feltthousands of miles away, he said.
"The object would be packing the equivalent of 40 billion tonsof TNT," said French. "That would be equal to about 40,000 hydrogenbombs all going off at the same time."
Henry Holt, a retired U.S. Geological Survey scientist nowworking on a NASA-funded project to detect and track asteroids thatcross Earth's orbit, found the asteroid in photographs taken almost aweek after it had flown past.
French said the fact that the asteroid, called 1989FC, was notseen earlier comes as no surprise.
"These detections are made because you can photograph a trail oflight as it moves against a background of fixed stars," French said."That means the asteroid is moving across your path and not on acollision course.
"An asteroid that was heading right for you would look just likeanother small fixed star and might be very difficult to detect untilit was too late to do anything about it," he said.
There have been several federally funded studies to determinehow to protect Earth from being bombed by outer space rocks, butlittle else has been done, French said. "Your best bet would be toidentify these several hundred or thousand Earth crossers (asteroids)and calculate their orbit," he said. "Then you would know when youare in danger."
Once the approaching object was sighted, French said, "You couldlaunch a large nuclear device to explode near the asteroid and nudgeit out of the way."
But, said the NASA scientist, "We don't really have the kind ofdetection capability or launch capability to do that at the moment."
French said Holt found the object in photos taken on March 31 byan 18-inch telescope at the Mt. Palomar Observatory in California.The photos were processed several days later, and the asteroid wasdiscovered by comparing views taken an hour apart.
By the time the object was photographed, French said, it hadalready made its closest approach to Earth and was streaking outward,away from the sun. Scientists plotted the trajectory backward anddetermined that the asteroid passed within a half-million miles ofEarth on March 23. That is closer to Earth than any other object solarge has passed since 1937, when an object called Hermes went byEarth at about the same distance.
French said that if 1989FC had struck dry land on Earth "itwould be a very major catastrophe for which we have had noexperience."
If the asteroid had hit the ocean, he said, it would havecreated tidal waves that would wash over vast areas of coastalregions.
French said there may be "hundreds to thousands" of suchEarth-crossing asteroids, and an object the size of 1989FC couldreasonably be expected to strike Earth every 5 million to 20 millionyears.
He said there is a crater in Ghana and one in the Soviet Unioncreated by such objects hitting Earth.
The Soviet crater is thought to be less than a million yearsold.
Scientists also believe that an object about 6 miles in diameterhit the Earth about 65 million years ago and ejected so much debrisinto the atmosphere that it caused the planet to cool.
A popular theory blames this event for the extinction of thedinosaurs.
French said the asteroid is moving away from the sun now, butwill pass within 20 million miles of the Earth early in October.
Experts are tracking the asteroid to gain a more refinedestimate of its orbit. French said 1989FC passed close enough to theEarth for its orbit to be affected by the planet's gravitationalinfluence. Scientists will have to take this into account whencalculating the asteroid's new orbit.
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